The present invention relates to a level shifter control circuit that selects different level shifters to perform level shifts of different sizes.
The invention is useful in, for example, an integrated circuit having circuit blocks that operate at different power-supply potentials, at least one of which is variable. One example is an integrated circuit having internal circuit blocks operating at a very low internal power-supply potential, and interface circuit blocks operating at an external power-supply potential that is normally much higher than the internal power-supply potential, but is reduced to a level near the internal power-supply potential when the integrated circuit operates in a low-power mode. Logic signals output by the internal circuit blocks must be up-shifted for input to the interface circuit blocks; that is, their high logic level must be raised from the internal power-supply potential to the external power-supply potential. The level shifters that normally perform this job have a wide operating range, but consume considerable current during signal transitions. In the low-power mode, it is desirable to use low-power level shifters that have a narrower operating range but consume less current.
The integrated circuit accordingly includes two different types of level shifters, a wide-range type and a low-power type, and has a level shifter control circuit that selects one type of level shifter or the other according to the difference between the internal and external power-supply potentials. The wide-range type is selected when the difference is large; the low-power type is selected when the difference is small.
A switchover from the low-power type of level shifter to the wide-range type causes no problems, because the wide-range type is immediately able to perform the necessary level shift. In a switchover from the wide-range type to the low-power type, however, the low-power type of level shifter may be initially unable to operate, because of the still-large difference between the two power-supply potentials. There may accordingly be a hiatus during which communication from the internal circuit blocks to the interface circuit blocks is interrupted, until the external power-supply potential falls to within the operating range of the low-power level shifters.
A more detailed description of this problem will be given in the detailed description of the invention.